Stack gases from many facilities, such as those of enhanced oil recovery processes, refinery hydrosulferization recycle, Claus sulfur recovery, etc., conventionally contain significant amounts of H.sub.2 S and CO.sub.2. These impurities act as very weakly ionized acids. It is conventional practice to use certain high boiling-point amines, which act as a mild alkali, to scrub these materials from the stack gases. Typically the amine is an alkanolamine such as ethanolamine, diethanolamine, methyl diethanolamine, diisopropanol amine and triethanol amine. In this scrubbing of the gases, the amines remove the H.sub.2 S and CO.sub.2. While these substances can be removed from the amines quite readily by conventional thermal recycling, other impurities are present in the amines after the scrubbing process either from the stack gas or from various chemical reactions. These other impurities form stronger acids that are sorbed by the amine irreversibly, at least by thermal regeneration. These are therefore called thermally stable anions and, in approximate decreasing order of concentration, are formate, acetate, thiocyanate, chloride, and sulfate. As referred to hereinafter, these are designated as Fo, Ac, CNS, Cl and SO.sub.4.
Ion exchange methods have been found to be generally effective for removing residual anions from amine stack scrubbers. Two typical ion exchange methods are described in U.S. Pat. Nos. 4,795,565 issued to T. Y. Yan on Jan. 3, 1989, and 4,970,344 issued to A. E. Keller on Nov. 13, 1990. Both of these methods utilize fixed bed ion exchangers. However, this fixed bed practice represents heavy ion exchange loading and involve high consumption of regenerants. For example, about one volume of feed completely loads one volume of resin. This demands very frequent regeneration or very large ion exchange beds. Thiocyanate anions are especially aggravating because of their very high affinity for the ion exchange resin. As a result, both of the above-cited references disclose two different anion resins, Type I and Type II, used in combination in order to counteract the effects of different resin affinities and different regenerant consumptions. Even with the use of two types of ion exchange resins, significant quantities of sodium hydroxide and sulfuric acid are required for the removal of the heat-stable anions including thiocyanate.
It is therefore an object of the present invention to provide a process for the regeneration of amines used for stack gas scrubbing that utilizes a single type of ion exchange resin which overcomes problems of resin loading and excessive regeneration reagent consumption.
It is another object of the present invention to utilize continuous countercurrent ion exchange, in contrast to fixed bed ion exchange, to overcome problems of resin loading during the regeneration of amines used in the scrubbing of stack gases.
A further object of the present invention is to utilize continuous countercurrent ion exchange, together with a selected chemical reaction to destroy thiocyanate, to overcome problems of resin loading and thiocyanate affinity during the regeneration of amines used in the scrubbing of stack gases.
These and other objects of the present invention will become apparent upon a consideration of the following description of the continuous countercurrent ion exchange method.